The invention is related to the physical phenomenon of surface plasmon resonance (SPR).
When light incident on a conductive surface (such as a thin layer or film of metal, typically gold or silver) possesses the proper wavelength, polarization and angle of incidence, the conducting electrons in the metal film oscillate in a resonant manner. This oscillation, together with the associated photon (or polariton electromagnetic (EM) wave) is called a surface plasmon wave. A surface plasmon is a surface electromagnetic wave that propagates along the surface of the metal film. A surface plasmon may be treated as a “quasi-particle”, that is, a hybrid of the photon and the electron waves in the material.
The surface plasmon wave absorbs the incident optical energy at the previously mentioned proper wavelength, polarization, and angle of incidence. Because of this absorbance, a radiation sensor may make use of the surface plasmon wave phenomenon. Where incident radiation covers a range of wavelengths, polarizations, and/or angles of incidence, the radiation reflected off the sensor's conductive surface demonstrates an anomalous reduction, or “dip,” at the particular values at which the surface plasmon resonance takes place.
Where a layer of material to be analyzed (hereinafter “analyte,” “adsorbed material” or “adsorbate”) is adsorbed onto the conductive surface, SPR provides an optical method for measuring the refractive index of the adsorbate. The measured refractive index may be used to calculate the mass, thickness, etc., of the adsorbed material. This SPR technique exploits the fact that, at the conditions specified above, surface plasmons on a metallic surface can be excited by photons, thereby transforming the photon into a surface plasmon. The conditions for producing such surface plasmons depend, in part, on the refractive index of the adsorbate.